The present invention relates to a light reflector, and in more detail, a light reflector for reflecting light from a light source to thereby achieve a high luminance.
Backlight type liquid crystal displays provided with built-in light sources have a wide market. As one example of such backlight type displays, a typical constitution of a side-light type display unit is shown in FIG. 1. The unit has a light-guide plate comprising a transparent acryl plate 3 having on one surface thereof a dot-printed plane 2, a light reflector 1 placed opposing to the dot-printed plane 2, a diffusing plate 4 placed opposing to the opposite plane of the light-guide plate, and a cold cathode lamp 5 placed in the lateral vicinity of the light-guide plate. In such constitution, light introduced into the light-guide plate from the lateral side thereof causes light emission at the dot-printed plane 2, to thereby prevent reflection or leakage of the light, which allows the diffusing plate 4 to produce a uniform surface emission of the light.
In such backlight unit, the light reflector functions so as to make an effective use of the light from the light source for display, and thus allows the displays to be adapted to the individual purposes. Since glaring mirror reflection is not preferred in general for the display, and instead it is necessary to provide by scattering reflection a relatively uniform surface luminance to thereby create a natural sight for the user. In particular for the light reflector for use in a liquid crystal display of the side-light type, it is necessary to uniformly reflect the light which otherwise tends to leak backward through the light-guide plate.
It is known from the past to add a white pigment such as titanium oxide or fluorescent brightener to a film composing the light reflector in order to raise the luminance thereof. It is also known to coat a white pigment such as titanium oxide on a metal plate such as aluminum plate to prevent the light transmission and mirror reflection.
As is described in the above, the conventional light reflector has been controlled in the optical functions thereof, such as luminance, through the use of components having optical properties. The present invention, however, shifts a point of view from such use of the optically functional material to the employment of a specific structure of the light reflector per se, to thereby improve the luminance at low cost.
The present inventors got an idea that, when the light introduced from the lateral side of the light-guide plate is refracted or reflected by the light reflector, a highly efficient reflection will be ensured and thus the luminance can be improved if the light reflector has a number of micro-lenses aligned so as to cover the entire surface thereof, where each micro-lens has an approximately exact circular profile. The present inventors found out that the foregoing problem can be solved at low cost if the function of such micro-lenses are assigned to voids produced in a stretched film, and that a light reflector having a luminance of 1,200 cd/m2 or above can uniformly reflect the light, which otherwise leaks backward through the light guide plate, to thereby achieve surface emission, which led them to complete the present invention.
That is, the present invention is to provide a light reflector comprising a biaxially stretched film containing a polyolefinic resin and a filler, wherein said filler is an inorganic filler having an average grain size of 0.1 to 8 xcexcm and/or an organic filler having a mean dispersion grain size of 0.1 to 8 xcexcm, said biaxially stretched film has an area stretched factor of 16 to 80, and said light reflector has a luminance of 1,200 cd/m2 or above.
A volume ratio of the filler to the biaxially stretched film is preferably 3.0 to 35% by volume, and for the case the inorganic filler is used, such inorganic filler preferably comprises calcium carbonate grains having a specific surface area of 20,000 cm2/g or above and excluding those with a grain size of 10 xcexcm or above. A ratio LMD/LCD, which is a ratio of stretching factor of said biaxially stretched film in the moving direction LMD to a stretching factor in the crossing direction LCD, is preferably 0.25 to 2.7. The porosity is preferably 15 to 60%, the opacity measured according to JIS (Japanese Industrial Standard) P-8138 is preferably 90% or above. The biaxially stretched film preferably has a multi-layered structure, and may have protective films on the front and/or back surface thereof.
It should now be noted that numerical ranges expressed with xe2x80x9ctoxe2x80x9d in this specification include both end values given before and after xe2x80x9ctoxe2x80x9d as minimum and maximum values, respectively.